Conventionally, “point-to-multi-point” access optical communication systems called passive optical network (PON) systems have been widely employed as systems capable of realizing public networks using optical fibers.
A PON system includes one optical line terminal (OLT) which is a station-side apparatus, and optical network units (ONUs) which correspond to a plurality of subscriber terminal apparatuses and are connected via an optical star coupler to the OLT. Currently, the PON systems have been actively introduced in order to realize broadband networks from the following merits: a major portion of optical fibers corresponding to transmission paths can be commonly used with the OLT by a large number of ONUs, and hence lower operation cost can be expected; an optical star coupler corresponding to a passive component has no need for power feeding and thus can be easily set outdoors; and reliability thereof is high.
For instance, in Gigabit Ethernet-passive optical network (GE-PON) having a transmission speed of 1.25 Gbit/s, which is standardized in IEEE 802.3ah, in a downlink from an OLT to ONUs, there is employed a broadcast communication system using an optical wavelength of 1.49 μm band, and the respective ONUs derive only data of the allocated time slots. On the other hand, in an uplink from the respective ONUs to the OLT, an optical wavelength of 1.31 μm band is employed, and a time division multiplexing communication system is employed by which transmission timing is controlled in order that data of the respective ONUs do not collide with each other.
In uplink communication of the above-mentioned PON system, the respective ONUs are located at different distances from the optical star coupler, and hence reception levels of the respective ONUs in the OLT are different for every reception packet. As a result, a wide-dynamic range characteristic capable of stably reproducing packets which have different light receiving levels is required for a receiving circuit of the OLT. As a consequence, generally speaking, there is provided an automatic gain control (AGC) circuit, which changes a conversion gain in response to a light receiving level, in a receiving circuit of an OLT, and the AGC is required to make high-speed response with long consecutive identical digit signals compatible.
Various types of AGC circuits have been proposed. For example, according to Patent Document 1, a feedback resistance value of a preamplifier, that is, a conversion gain thereof is switched stepwise based on a light receiving level. According to Patent Document 2 which is a conventional technology referred to in Patent Document 1, the feedback resistance value of the preamplifier, that is, the conversion gain thereof is changed in an analog manner based on the light receiving level.
Patent Document 1: JP 2000-151290 A
Patent Document 2: JP 07-38342 A